North American Network Operators Group

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Re: MTU of the Internet?

  • From: Frank Kastenholz
  • Date: Mon Feb 09 11:04:55 1998

At 09:11 PM 2/7/98 -0800, Sean M. Doran wrote:

>Tli was just pointing out n messages ago that no matter
>how well you do in terms of aggregating data traffic into
>bigger chunks, you still will see an enormous number of
>small packets around (ACKs).  You have to be prepared to
>switch those at line rate; engineering for some
>statistical mix of big and small packets is asking for a
>disaster when someone suddenly goes simplex.

some of the histograms i've seen show close to 50%
of the packets being 40 bytes long. the 'desired'
tcp behavior is to have no more than 2 data packets
for every ack (since congestion control uses ack-reception
to pace the transmission of data and try to 
quickly detect losses).

>There is, however, the spectre of there being so many SYNs
>flying around that they alone might cause congestion
>collapse.  I dunno if I should be frightened of that or

you should be.
not because of the packet-load it causes (as tony pointed out,
you have to be able to move 40-byte packets at 'fiber speed')
but because it's a symptom of lots of short-lived tcp connections.
these connections never get out of slowstart. when there is only
a small number of them, it's not important. when there is a large
number of them, you have large, non-congestion-controlled, data flows.
it's called being nibbled to death by mice.